If your fans don’t move at least 3,000 cfm they’re not enough, I don’t care what kind of radiator you have. There are really expensive fans out there that don’t move enough cfm. The Ford Contour fans I run will flow 3,500 cfm on the low speed setting and 5,000 cfm on high speed, that’s the ballpark you need to be in for electric fans on the street. Less than that won’t keep up.
You said you had march pulleys, whats the drive ratio on that water pump pulley? If you’re under driving the water pump too much it’s not gonna keep up. Most performance pulleys are under drive pulleys for more horsepower, they’re not made for cooling. You could have the “right” pulleys, but still have an under driven water pump. It’s all about application.
How many blades on your water pump? There are standard and high flow versions, they were designed for different pulley ratios.
The freeze plug blowing out it interesting. Might have a plugged passage in the block, although it could also just be a loose plug. I would suspect other issues first (like the fans not moving enough cfm). The cylinder temps will always be higher on the center two cylinders. If the right and left backs are different that could be a block machining deal, if it wasn’t square decked your compression ratio’s may actually vary side to side. Could also be a fuel issue, but if you’re TBI it shouldn’t be.